Contraction or “beating” of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation or “AF.” Certain cardiac arrhythmias can be treated by deliberately damaging the tissue of the cardiac wall along a path crossing a route of abnormal conduction. This causes formation of a scar extending along the path where disruption occurred. The scar blocks conduction of the electrical impulses. Such a scar can be created by conventional surgery, but this entails all of the risks and expense associated with cardiac surgery. Another approach, described in Swartz et al., U.S. Pat. No. 5,575,766, is to introduce a catheter bearing a localized energy emitter such as an electrode for application of radio frequency (“RF”) energy at its distal tip into a heart chamber, such as the right or left atrium of the heart in the case of atrial fibrillation. The physician then moves the catheter so that the tip, and the localized emitter traces the desired path. In AF, the desired path typically is a closed loop encircling the openings or ostia of the pulmonary veins. RF energy applied through the electrode heats the tissue to a degree sufficient to cause death of the normal tissue and its replacement by scar tissue. Heating to this degree is referred to herein as “ablation.” Typically, heating to about 60-80° C. is sufficient. Tracing a precise path along the interior of a chamber in the heart of a living subject with the tip of a catheter involves inherent practical difficulties. Although curved guide wires can be placed within the catheter so that the catheter tip will tend to follow the guide wire as the physician moves it, the process is still difficult.
Swanson et al., U.S. Pat. No. 5,582,609 describes an elongated catheter having numerous RF electrodes disposed along its length in a distal region adjacent the tip. This distal region can be formed into a curved, loop-like configuration and manipulated so that the electrodes lie along the desired path, whereupon RF energy is applied so as to ablate cardiac tissue. In a variant of this approach, the electrodes are mounted on a structure which opens to form a ring-like configuration. Even with these structures, however, it is difficult to assure the desired placement of the RF electrodes. Lesh, U.S. Pat. No. 5,971,983 describes an elongated catheter which is equipped with similar RF electrodes distributed over its distal region, and uses guide wires to position the distal region in place against the wall of the heart. Although this patent mentions an “ultrasonic element such as an ultrasound crystal element” along with numerous other devices as theoretically applicable to cardiac tissue ablation, it offers no structure for an elongated ultrasonic ablating device.
As described in Lesh, International Publication WO 99/02096, the abnormal conduction routes in AF typically extend from the wall of the heart along the pulmonary veins. Therefore, AF can be treated by ablating tissue in a ring around each pulmonary vein at the juncture between the pulmonary vein and the heart. As described in the '096 publication, such ablation can be performed by threading a catheter having a thermal ablation element at its distal tip into the heart so that the tip is lodged within the appropriate pulmonary vein. The catheter may bear a balloon which is inflated within the vein and which holds the catheter in place. The ablating element is then actuated so as to apply heat in a region surrounding the ablating element. In certain embodiments taught in the '096 publication, the ablating element includes a radio frequency (“RF”) emitting element which is carried on the surface of the balloon. Ablation of the pulmonary vein using RF energy can create a rough, disrupted surface on the interior of the vein. This or other factors can lead to stenosis of the pulmonary vein or thrombosis, i.e., formation of blood clots.
Other embodiments described in the '096 publication disclose the use of ultrasonic transducers. The preferred ultrasonic transducer illustrated in the '096 publication is a rigid ceramic piezoelectric element disposed on a catheter surrounded by a balloon. When the balloon is inflated, the piezoelectric element remains remote from the wall of the pulmonary vein. The piezoelectric element can be actuated to apply sonic energy through a fluid contained in the balloon, thereby heating the ring of vein wall tissue surrounding the balloon. As a further alternative, the '096 publication shows an ultrasonic emitter in the form of a hollow concave disk. The '096 publication suggests that such an emitter can be physically rotated around the axis of a catheter so as to ablate a ring-like zone. These transducers have numerous drawbacks even for use in ablation of a vein wall and are not adapted for ablation of the wall of the cardiac chamber.
Ultrasonic heating such as high intensity focused ultrasound (HIFU) is utilized for certain therapeutic applications. As disclosed in commonly assigned International Application PCT/US98/1062, published as International Publication WO/98/52465 the disclosure which is hereby incorporated by reference herein, HIFU heating typically is conducted using an ultrasonic emitter having an array of transducers. The transducers are actuated with a drive signal so as to emit ultrasonic waves. The relative phasing of the waves is controlled by the physical configuration of the array and the phasing of the drive signal. These factors are selected so that the ultrasonic waves tend to reinforce one another constructively at a focal location. Tissue at the focal location is heated to a greater extent than tissue at other locations. As described, for example in U.S. Pat. Nos. 6,461,314 and 6,492,762, the disclosures of which are also incorporated by reference herein, HIFU may be applied by transducer arrays such as arrays of polymeric piezoelectric transducers. These arrays can be mounted on a probe such as a catheter which can be introduced into the body as, for example, within the vascular system or into a cavernous internal organ. The '314 patent discloses certain transducer arrays which can be deformed so as to vary the placement of the focal location.
Despite all of these efforts in the art, there have been needs for further improvements in the devices and methods used to apply thermal energy to the cardiac wall for treatment of atrial fibrillation, particularly the need to tightly control the zone of damage to cardiac tissue in order to minimize collateral damage to neighboring tissues. There have been corresponding needs for further improvement in the devices and methods used to apply energy to other organs of the body for thermal treatment.